Active vibration actuators, like passive vibration absorbers, generally consist of two separate mass portions, one of which is typically attached to a target region for suppression of vibrational disturbance while the other is suspended so that it can vibrate in a manner to reduce the vibrational disturbance. In an active vibration actuator a suspended mass is driven to vibrate, typically electromagnetically, while in the passive vibration absorber the vibrating mass receives drive excitation only through reaction between the two masses and thus the vibrational disturbance can only be attenuated, never fully cancelled.
In an electromagnetic active vibration actuator, the two masses typically correspond to a stator assembly and a vibratable armature assembly, either or both of which can include a coil powered from an AC (alternating current) electrical source and/or a permanent magnet system; a suspension system between the two mass portions allows reciprocal vibration, which takes place at the frequency of the applied AC. Generally the stator will be solidly attached to a machine, engine frame or other body subject to vibrational disturbance, while the armature is vibratably suspended and is driven to vibrate, relative to the stator, at a predetermined frequency, typically that of the vibrational disturbance, the phase and amplitude being optimized to produce a counter-reaction from the driven vibrating armature mass that act in a manner to suppress the vibrational disturbance.
Another version of active vibration actuator delivers output via a moving shaft, typically driven axially; the main body of the actuator unit is attached solidly to a massive body such as a machine frame, and the output shaft is attached to the part or region in which vibrational disturbance is to be suppressed by transmitting a counteracting vibrational force via the output shaft.
Theoretically, a non-feedback active vibration control actuator could be fine-tuned and adjusted in manner to completely cancel disturbing vibration, however in order to track any change that may take place in the parameters of the vibration, the active vibration actuator is usually placed under control of a feedback loop that responds to sensed vibration.
Typical structure of an active vibration control actuator is coaxial, with the stator assembly including a soft steel tubular shell housing surrounding an axially-vibratable armature assembly. The stator assembly and/or the armature assembly can include any of three basic elements: permanent magnets, coils and/or low-reluctance path segments such as yokes, cores, pole pieces, etc. made from ferromagnetic material such as soft steel or iron. Such magnetic material will be referred to henceforth herein simply as iron.
Such actuators are motivated via magnetic flux paths that can each be represented by a loop that typically includes at least a coil, a permanent magnet, one or more iron segments and one or more relatively small air gaps.
This mass is motivated electromagnetically from AC in the coil in a manner to cause it to vibrate at frequencies, amplitudes and phase angles that optimally suppress the disturbing vibration: this may be accomplished by an electronic feedback loop and control system that senses vibration both at its source and in the disturbed region, and automatically adjusts the frequencies, amplitudes and phase angles to minimize the disturbing vibration.
Typically the vibrating mass is supported by end spring suspension members or flexures which act to hold it centered when in a quiescent condition, i.e. with no current applied to the coils. The mechanical spring is characterized by a spring modulus (sometimes referred to as spring constant or spring rate) defined as force/deflection distance. The combination of the spring modulus and the vibrating armature mass determines a frequency of natural vibration resonance. Current in the coil(s) of the actuator generally acts in a manner of a negative spring modulus to override the force of the mechanical spring suspension and drive the armature to vibrate at the driven frequency; however, at frequencies other than the natural resonant frequency, the actuator may operate inefficiently due to improper magneto-mechanical coupling.
Overall electrical power efficiency, i.e. mechanical output energy versus electrical driving power, is important in an active vibration control actuator; the different configurations of the basic elements found in known art represent different approaches seeking to optimize the important overall parameters such as efficiency, performance, reliability and ease of manufacture. A key factor is the natural mass-spring resonance and the extent to which this can be altered or overpowered by the electromagnetic drive system.
Active electromagnetic vibration control actuators of known art can be categorized in two general types: voice coil type and solenoid type.
The voice coil type of actuator gets its name from well known loudspeaker structure wherein a tubular voice coil assembly, typically a single layer of wire on a vibratable voice coil form, is constrained concentrically by suspension means and centered in an annular magnetized gap of constant separation distance and constant permeability formed in a flux path loop that includes a stationary permanent magnet. When an electrical current is applied to the voice coil, a force equal to the cross-product of current and magnetic flux density is exerted on the voice coil in a direction defined by the classical Right Hand Rule of electromagnetics, driving the voice coil in the direction of the force to a displacement that is constrained by the suspension springs.
Typically the loudspeaker voice coil is made to extend well beyond the region of the magnetic gap symmetrically in both directions, so that at any instant, as it travels back and forth, only that portion of the voice coil within the magnetic gap interacts directly with the concentrated magnetic field to produce the driving force. Alternatively the voice coil may be made much shorter than the extent of the magnetic gap so that, when vibrating to its limit of travel, it remains entirely within the magnetic gap. In either case, in the conventional loudspeaker voice coil driver, there is an inherent sacrifice of efficiency due to this partial coil-to-magnet coupling, in a tradeoff to gain linearity and long stroke travel capability.
In applying the voice coil principle to active vibration actuators, generally the fixed portion or stator is made to include a tubular iron shell housing. The voice coil may be made multi-layer, may be associated with nearby iron members for concentrating flux and may be made fixed rather than moving. The typical fixed central magnetic core pole piece of the loudspeaker may be replaced by a movable central armature suspended in a manner to be vibratable axially, usually constrained by end springs, thus constituting a vibratable mass.
In a moving-coil version of a voice-coil type actuator, permanent magnets may be attached immediately inside the fixed iron outer shell stator assembly surrounding a vibratable armature which carries multi-layer coils wound on a iron core formed with associated iron pole-piece prominences, and which thus constitutes the vibratable mass.
Conversely, in a moving-magnet version of a voice-coil type actuator, multi-layer coils may be attached immediately inside the iron outer shell stator assembly, surrounding the vibratable armature which carries permanent magnets and associated iron pole-piece prominences, and which thus constitutes the vibratable mass.
Typically, in both the moving-coil and the moving-magnet versions of voice-coil type active vibration actuators, a concentric central moving armature is configured with at least two magnetic prominences formed by short cylinders whose circumferences each form an annular magnetic air gap with the iron shell. In typical cross-section, the armature prominences and the stator prominences are made to both face a common reference line from opposite sides so that the armature assembly can be easily inserted into and withdrawn from the stator assembly.
Electromagnetic active vibration actuators can be classified into two general types: voice-coil type and solenoid type. Both types may have a coaxial electromagnetic structure wherein a stator portion and an axially-vibratable armature are linked together by a magnetic flux loop path that includes at least one permanent magnet, an AC-driven coil, and at least one magnetic air gap.
The voice coil type operates on the principle of force acting on wire in a coil in a magnetic field, the force acting in a direction perpendicular to the direction of current and perpendicular to the magnetic field, according to the Right Hand Rule. The magnetic field is concentrated in an air gap (or gaps) having a separation distance and permeability that remain substantially constant in operation as the armature travels axially. The armature, like the voice coil of a loudspeaker, requires some form of spring suspension to establish a normal stabilized centered position, otherwise the armature would free-float axially and drift off center.
In contradistinction, the solenoid type actuator operates generally on the principle of attraction between movable magnetized bodies; more particularly a magnetic force acts on a movable armature through a magnetized air gap whose separation distance varies with armature displacement and thus the permeability is incremental, the armature tending to move in a direction that intensifies the magnetic flux in the air gap.
A simple solenoid without any permanent magnet typically attracts an armature from an offset large-gap position to a centered small-gap position or an end-of-travel closed-gap position in response to DC of either polarity in the coil; thus, with AC applied to the coil, any vibration response would be very inefficient and at a doubled frequency. For use as a vibration control actuator, the solenoid is modified to be magnetically biased, e.g. by the addition of a pair of permanent magnets (or one permanent magnet and a second coil) to form a dual-gap solenoid type actuator.
When the coil of such a dual-gap solenoid type actuator is AC-driven, thus vibrating the armature, there is a recurring redistribution of magnetic flux in each pair of gaps that sets up eddy currents in the pole pieces. Therefore, while the dual-gap solenoid type provides good efficiency, especially in applications where the armature may be allowed to travel to an end limit where the gap closes, in active vibration applications the dual-gap solenoid type generally suffers the disadvantages of complexity of structure and the need for tight tolerances between parts. Another disadvantage is the limitation of the amplitude of travel of the armature, limiting the use of this type of actuator to high frequencies. At such high frequencies, the iron pole pieces may require slotting or lamination to avoid excessive eddy current losses due to the magnetic flux variations. Yet another disadvantage is the small mass of the armature, making it usually necessary to use the exterior mass of the coil and magnet structure as the inertial mass. Also, while a voice coil type actuator can be readily extended by adding more voice-coils and corresponding gaps, the dual-gap solenoid type actuator can be extended only by adding one or more complete similar actuator units in a tandem manner.